This invention is directed to semiconductor devices and methods of making such devices, and more particularly to an improved resistor element for MOS integrated circuits.
In the earliest semiconductor integrated circuits, resistors were provided by diffused regions or by portions of the semiconductor substrate which were defined by etching, as seen in U.S. Pat. No. 3,138,743 issued to Jack S. Kilby and assigned to Texas Instruments. As the complexity of integrated circuits grew, the area occupied by resistors became more and more prohibitive, so logic forms were favored which did not require resistors. For example, "TTL" or transistor-transistor logic in bipolar was standard in digital equipment for a long period, and one of the features of TTL was minimizing the area on a bar dedicated to resistors. Integrated injection logic or I.sup.2 L is a more recent bipolar form which does not need resistors. In MOS logic and memories, transistors are used as load devices or in other places where resistors are conventionally required. Examples of very complex MOS circuits containing many thousands of transistors but no resistors in a single chip digital processor or memory are shown in U.S. Pat. No. 3,940,747, issued to Kuo and Kitagawa and U.S. Pat. No. 3,900,722, issued to Michael J. Cochran et al, both assigned to Texas Instruments.
High density MOS memory devices such as the 4096 bit memory described in U.S. Pat. No. 3,940,747, or the "16K" or 16384 bit memory described in pending application Ser. No. 682,687, filed May 3, 1976, by N. Kitagawa, now abandoned, have been of the dynamic type because dynamic cells are smaller in area. In some parts of digital equipment, however, the refresh circuitry required for dynamic memories is incompatible, so static memory is used. Static cells traditionally employ six-transistor bistable or flip-flop circuits wherein depletion-load MOS transistors are used as load devices. These cells are much larger than the one-transistor cells of dynamic memory devices, so the density is less. Also, power dissipation is high due to the requirement that some current must flow through one side of each cell in the array to maintain the stored data.
In co-pending application Ser. No. 691,252, filed May 28, 1976, assigned to Texas Instruments, there is disclosed a resistor element particularly for a static RAM cell wherein the resistors are implanted regions buried beneath field oxide. With the resistor element of this invention, access time and layout are improved.
It is a principal object of this invention to provide improved resistor elements for integrated circuits. Another object is to provide an improved static RAM cell for MOS memory devices. An additional object is to provide small area, high resistance load elements for transistors in semiconductor integrated circuits.